The invention relates to a ceramic material, especially an electrically conductive ceramic material, as well as its production and use.
Electrically conductive ceramics are typically used as connecting elements between components. These ceramics ensure on the one hand an electrical contact between two components with which these connecting elements have as a consequence of a joining process an intimate material to material contact. On the other hand electrical conductive paths can be formed from such ceramics and which are located in or on a component and either as a consequence of an applied electric current can induce a physical-chemical change in the component (actuator function) or can transform a change of a physical-chemical characteristic of the component into an electrically measurable magnitude (sensor function).
From the literature a series of manufacturing processes are known which give rise to being sintered at low temperatures. Very often synthesis methods have involved sol-gel intermediate products or nanophasal powders to lower the sintering temperatures of ceramics. Such processes are characterized by the fact that they require very expensive (frequently metal-organic) starting materials and auxiliary materials and produce only very small quantities, see for example U.S. Pat. No. 4,636,378.
Another method of obtaining low sintering ceramics is known from EP 0 280 033 B1. In this case, precipitation reactions are carried out with, for example, hydroxides or oxalates. These have the drawbacks that the salts which are frequently formed, tend to separate out in complex compositions on the one hand or because of different solubility products of the salts precipitate only incompletely, and can thus give rise to deviations from stoichiometry of the products. A further disadvantage is frequently the need for organic solvents or purification agents which increase the cost of the fabrication process.
Furthermore, from U.S. Pat. No. 3,330,697, a fabrication process for niobates, zirconates and titanates of lead and alkaline earth metals is known. In this case, into a solution of polyhydroxy alcohol and citric acid compounds of titanium, zirconium and niobium are mixed with lead or alkaline earth salts. By heating the solution, the organic components are removed. This method of production is known in the literature as the Pechini method. This method is however not suitable for ceramics sintering at low temperatures as described in the patent. The polyhydroxy alcohols which are there used have the disadvantage that the basic solution with the cations and the citric acids, upon heating, is transformed into a viscous resin. In addition, the higher proportions of organic auxiliary materials can give rise to a spontaneous combustion of the viscous resin with further temperature elevation in the examples given. Investigations of this fabrication process as well as of other fabrication processes which are based on uncontrollable ignition of the intermediate product have shown a significant limitation of the sinterability of the ceramic powder which is obtained. A heating like that which results from the combustion of the resin is thus to be strictly avoided in the fabrication of being sintered at low temperature.
The object of the invention is to obtain a ceramic material which, by comparison with the state of the art, has improved sinterability and reduced sintering temperature. In addition the ceramic material should have good electrical properties. Further it is an object of the invention to provide a fabrication process for such a ceramic material.
With the method according to the invention of claim 1, at least two different metal nitrates or metal carbonates are dissolved in an aqueous solution together with a metal complex former, concentrated, and at low temperature converted to a solid.
The metal can be at least one metal from the first group Axe2x80x2=(Y, Sc, Ce, La, Pr, Nd, Sm, Eu, Gd) and at least one metal from a second group B=(Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, W, Sn, Sb, Pb, Bi).
As the metal complex former, any complex former which is soluble in water is suitable which can complex with the above-mentioned metals and bring them as metal ions into solution. The metal complex formers which are to be counted in this group are especially the lactic acids, the citric acids, the citric acid esters or also the tartaric acid complexers. Furthermore, suitable are also other polycarboxylic, polyhydroxycarboxylic acids or polyaminocarboxylic acids like, for example, EDTA, (ethylenediamenetetraacetic acid).
The solution of metal complex former and metal salts is heated in such a way that the metal complex former decomposes. This can be achieved by for example splitting off gaseous carbon monoxide or carbon dioxide (CO, CO2) or by splitting off also gaseous nitrogen oxides (NOx). By means of the heating, water is simultaneously evaporated from the aqueous solution so that the initially dissolved or complex metal ions form a solid.
In an advantageous embodiment of the method, at least on metal of the group Axe2x80x3=(Mg, Ca, Sr, Ba) is dissolved as a nitrate or as a carbonate in the aqueous solution.
In an advantageous embodiment of the method, the metal compounds are used in a predetermined ratio. To that effect, the stoichiometry of the metals in a ceramic of the following compositions should be met: ABO3, A2BO4 or A2B2O7 where A stands for the elements of the mentioned groups Axe2x80x2 and Axe2x80x3 and B for the mentioned elements of group B.
It has been found to be advantageous further when, with the method according to the invention, different metals of group B are used in different proportions. In this case, especially the following compositions have been found to be especially desirable:
The compositions indicated under a) to k) are advantageously suitable for use in fuel cells while the compositions described under l) to p) are advantageous for use in piezoceramics.
The heating of the aqueous solution is carried out especially slightly initially until the major part of the water is evaporated. Then the temperature is increased further, especially to up to 700xc2x0 C. At these temperatures the materials according to the invention are transformed advantageously into a Perovskite or a multiphase ceramic which has as a major component a Perovskite.
The ceramic material according to the invention of claim 8 is produced by the method of the invention and has the following composition:
Axe2x80x21-x-yAxe2x80x3xBxe2x80x21-a-bBxe2x80x3aBxe2x80x2xe2x80x3bO3 
with
Axe2x80x2=(Y, Sc, Ce, La, Pr, Nd, Sm, Eu, Gd)
Axe2x80x3=(Mg, Ca, Sr, Ba)
Bxe2x80x2=(Mn, Fe, Co)
Bxe2x80x3=(Ti, V, Cr, Ni, Zn, Pb, Sb, W, Zr)
Bxe2x80x2xe2x80x3=(Cu, Bi)
x=0-0.6 y=0-0.2
a=0-1 b=0-0.8
The material is electrically conductive and usually has a significantly improved sinterability by comparison with conventional ceramics. By an improved sinterability is to be understood a lower temperature range for the sintering which can be in the range from 800xc2x0 C. to significantly below 1000xc2x0 C. In addition, such materials have a smaller grain size distribution than those materials which are known in accordance with the state of the art. The typical order of magnitude for the individual particles (proportion greater than 80%) is in the range of 0.2 to 7 xcexcm with a mean particle diameter of 1 to 2 xcexcm. This smaller grain size usually results in a significantly improved homogeneity of the ceramic during the sintering process.
Furthermore, by suitable choice of the metals used a smaller thermal expansion coefficient in the range of 1xc3x9710xe2x88x926 Kxe2x88x921 to 16xc3x9710xe2x88x926 Kxe2x88x921, especially between 3.5xc3x9710xe2x88x926 Kxe2x88x921 and 13xc3x9710xe2x88x926 Kxe2x88x921 can be established in a close to optimal manner. This is especially the case for the exemplary compositions l), n) and p).
The establishment of thermal expansion coefficients is especially of advantage when the material according to the invention is to be used as a connecting element either for high temperature fuel cells or together with piezoelectrics or dielectrics. The thermal expansion coefficients of the latter materials lie in the range of about 3.5 to 6.5xc3x9710xe2x88x926 Kxe2x88x921. Layers with thermal expansion coefficients which are so matched give rise as a rule to fewer mechanical stresses than with conventional connecting elements like for example those composed of silver, palladium or platinum. These have in the same temperature interval a substantially higher expansion coefficient (22.5xc3x9710xe2x88x926 Kxe2x88x921 for Ag, 13.5xc3x9710xe2x88x926 Kxe2x88x921 for Pd and 9.8xc3x9710xe2x88x926 Kxe2x88x921 for Pt). Furthermore, the described exemplary compositions l), m) and n) contain the elements of piezo lectrics and dielectrics so that a very good chemical compatibility and reduced piezo and dielectric characteristics can be assured generally with improved long term stability of the ceramics.
Advantageously, the ceramics according to the invention, can be used for example in piezoelectrics (sensors or actuators), which as a rule are comprised of barium titanate or lead-zirconium-titanium-oxide (PbZr1-xTixO3, or for short PZT). Both of these classes of material are comprised, like the ceramics according to the invention, of the advantageous configuration of Perovskites.
If the material is in the form of a Perovskite, it has slight piezoelectric properties so that during a mechanical stress a component fabricated therefrom can produce reduced stresses between individual elements and thus contribute to a longer useful life of the component. The material according to the invention is further suitable especially for use as electrically conductive connecting elements, especially for metal-metal, metal-ceramic or ceramic-ceramic bonding.
A connecting element from the material according to the invention is, in addition, suitable for use in high temperature fuel cells (SOFC) in which these c ramics form a fixed junction between metallic bipolar plates and the cathode. These ceramic materials according to the invention can thus replace noble metals, like silver, palladium or platinum, as electrically conductive contacts. As a result there is an improvement over the state of the art alone in the reduced material cost.
Advantageously, the application and formation of junctions between PZT ceramics and the connection materials of the invention can be easily realized. If conventional PZT ceramics are used, these in general are sintered at temperatures of 1200xc2x0 C. A joining process at 800 to 1000xc2x0 C. can thus be achieved easily. Low temperature sintering PZT ceramics with sintering temperatures below 1000xc2x0 C. permit sintering of them together with the materials according to the invention and it is possible thereby to save one process step, i.e. a temperature treatment.